Golf ball cover composition

ABSTRACT

A golf ball cover is formed of a composition comprising (a) 70-95% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof and (b) 30-5% by weight of a sodium or zinc salt of an ethylene-unsaturated carboxylic acid copolymer, an ethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof, having an M.F.R. of at least 4, the composition curing into a product having a hardness of 55-80 on JIS C scale and a tensile stress at 10% distortion of 40-140 kg/cm 2 . The composition is easy to mold and allows for manufacture of a golf ball having improved spin properties and resistant to impact by iron and other clubs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a golf ball cover composition.

2. Prior Art

As the cover material of two-piece golf balls and some wound golf balls,ionomer resins in the form of ethylene-(meth)acrylic acid copolymer arewidely used and accepted because of their long-lasting impact resistanceand cut resistance. Since the ionomer resins provide hard hitting feeland high hardness as compared with the balata rubber conventionally usedas the cover material, golf balls with an ionomer resin cover are saiddifficult to impart a desired quantity of spin and inferior to controlon iron shots.

For improvements in these respects, U.S. Pat. No. 4,884,814 or JP-A308677/1989 proposes to blend an ionomer resin in the form of anethylene-(meth)acrylic acid copolymer having a certain spectrum ofphysical properties with a specific amount of an ethylene-(meth)acrylicacid-(meth)acrylate terpolymer ionomer which is a relatively softionomer resin. The soft/hard ionomer blend is used as a golf ball cover.This is a quite effective technique for improving the hitting feel andcontrol of golf balls using a conventional ionomer resin in the form ofan ethylene-(meth)acrylic acid copolymer as the cover.

Nevertheless, the golf ball cover of the above-mentioned soft/hardionomer blend has several problems. The cover is softer and allows theball to receive more spin on iron shots, which means the increasedfrictional force between the club face and the cover. Then balls using ahard core such as two-piece solid golf balls have the likelihood thatthe ball surface be peeled as a consequence of iron shots because thecover surface can be peeled by grooves across the iron club face.

An improvement in this regard is proposed in JP-A 277208/1993. A mixtureof two or more metal salts of an ethylene-unsaturated carboxylicacid-unsaturated carboxylate terpolymer having a low flexural modulus isused as the cover for the purpose of protecting the ionomer coveragainst fretting by iron club shots. This technique has many problems inmanufacturing or molding because of poor flow of the material usedtherein and the resulting balls are inferior in performance. Forexample, when a conventional cover enclosing process (involvinginterposition of a core between half cups of the cover material followedby heat pressure molding or injection molding of the cover materialaround a core) is used, the low flowing material has the problems thatas compared with smooth flowing resins, dimples on the inner surface ofa mold are poorly transferred thereto, that the mold configuration isnot faithfully reflected to exacerbate ball sphericity, and that weldlines appear to reduce impact durability.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide good feel,ease of spin or control, and resistance to peeling.

Making efforts to develop a golf ball cover composition which isflexible and readily moldable and which offers a soft feel, good spinproperty, and improved resistance to peeling by iron shots when used asthe golf ball cover, we have found that when a composition is comprisedof (a) 70 to 95% by weight of a sodium or zinc salt of anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymeror a mixture thereof and (b) 30 to 5% by weight of a sodium or zinc saltof an ethylene-unsaturated carboxylic acid copolymer, anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymeror a mixture thereof, having a melt flow rate (M.F.R., defined below) ofat least 4, the composition curing into a product having a hardness of55 to 80 on JIS C scale and a tensile stress at 10% distortion of 40 to140 kg/cm², there is obtained a golf ball cover which has overcome theabove-mentioned problems, offers improved flexibility, soft hittingfeel, susceptibility to spin, and ease of control, and is resistant toscuff and damages by iron clubs.

According to the present invention, there is provided a composition forforming a golf ball cover, comprising

(a) 70 to 95% by weight of a sodium or zinc salt of anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymeror a mixture thereof and

(b) 30 to 5% by weight of a sodium or zinc salt of anethylene-unsaturated carboxylic acid copolymer, an ethylene-unsaturatedcarboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof,having an M.F.R. of at least 4,

the composition curing into a product having a hardness of 55 to 80 onJIS C hardness scale and a tensile stress at 10% distortion of 40 to 140kg/cm².

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, the cover of the golf ball is formedof a composition comprising (a) a sodium or zinc salt of anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymeror a mixture thereof and (b) a sodium or zinc salt of anethylene-unsaturated carboxylic acid copolymer, an ethylene-unsaturatedcarboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof.

Component (a)

It is a sodium or zinc salt of an ethylene-unsaturated carboxylicacid-unsaturated carboxylate terpolymer. The unsaturated carboxylic acidis preferably selected from aliphatic monocarboxylic acids having 3 to 8carbon atoms, especially acrylic acid and methacrylic acid. Theunsaturated carboxylate is preferably selected from esters ofunsaturated carboxylic acids having 3 to 8 carbon atoms (e.g., acrylicand methacrylic acid) with alcohols having 1 to 9 carbon atoms, theesters having 4 to 12 carbon atoms in total. Exemplary esters includemethyl acrylate, ethyl acrylate, isobutyl acrylate, n-butyl acrylate,2-ethylhexyl acrylate, methyl methacrylate, and isobutyl methacrylate.

Examples of the sodium and zinc salts of terpolymer are shown in Table1.

                  TABLE 1                                                         ______________________________________                                                                       Flexural                                                                             M.F.R.                                                       Neutralizing                                                                            modulus                                                                              (g/10                                   Trade name                                                                             Manufacturer                                                                              metal     (psi)  min.)                                   ______________________________________                                        Himilan 1855                                                                           Mitsui -- DuPont                                                                          Zn        13,000 1.0                                              Polychemical                                                                  K.K.                                                                 Himilan 1856                                                                           Mitsui -- DuPont                                                                          Na        10,100 1.0                                              Polychemical                                                                  K.K.                                                                 Surlyn 9020                                                                            E. I. DuPont de                                                                           Zn        14,400 1.1                                              Nemours Co.                                                          Surlyn 8020                                                                            E. I. DuPont de                                                                           Na        14,400 1.0                                              Nemours Co.                                                          Surlyn 8120                                                                            E. I. DuPont de                                                                           Na         7,100 0.9                                              Nemours Co.                                                          Surlyn 8320                                                                            E. I. DuPont de                                                                           Na         2,800 0.9                                              Nemours Co.                                                          Iotak 7510                                                                             Exxon Chemical                                                                            Zn         5,200 0.8                                              Corp.                                                                Iotak 7520                                                                             Exxon Chemical                                                                            Zn         3,800 2.0                                              Corp.                                                                ______________________________________                                    

The sodium and zinc salts of terpolymers may be used alone or inadmixture of two or more. A blend of sodium and zinc salts is preferredfrom the standpoints of repulsion and durability. In such a blend, thesodium and zinc salts are desirably mixed in a weight ratio of from20:80 to 80:20, especially from 40:60 to 70:30.

It is seen from Table 1 that these sodium and zinc salts of terpolymersare relatively soft among ionomer resins since they have a flexuralmodulus of about 2,500 to 15,000 psi, indicating that they are suitableas soft feeling covers. However, their M.F.R. which is an index of flowis not more than 2.0, mostly not more than 1.0, indicating that they arelow flowing and difficult to mold.

It is to be noted that the melt flow rate (M.F.R.) as used herein ismeasured according to JIS K-7210 (1976) Method A.

Component (b)

Therefore, it is necessary to improve the flow of the sodium or zincsalt of terpolymer or mixture thereof. According to the presentinvention, to the sodium or zinc salt of terpolymer or mixture thereofis added component (b) which is smooth flowing and compatible with themetal salt of terpolymer. Component (b) is a sodium or zinc salt of anethylene-unsaturated carboxylic acid copolymer, an ethylene-unsaturatedcarboxylic acid-unsaturated carboxylate terpolymer or a mixtureComponent (b) should preferably have an M.F.R. of up to 30 because abovethis limit, the resulting balls would be deteriorated in durability(scuff resistance) and performance (repulsion).

In the sodium and zinc salts of ethylene-unsaturated carboxylic acidcopolymer, the unsaturated carboxylic acid is preferably selected fromaliphatic monocarboxylic acids having 3 to 8 carbon atoms, especiallyacrylic acid and methacrylic acid. These salts should have an M.F.R. ofat least 4. Salts with an M.F.R. of less than 4 are less effective forimproving moldability. Illustrative examples of the. sodium and zincsalts of ethylene-unsaturated carboxylic acid copolymer are shown below.

                  TABLE 1                                                         ______________________________________                                                               Flexural                                                                      modulus   M.F.R.                                       Trade name                                                                              Manufacturer (psi)     (g/10 min.)                                  ______________________________________                                        Himilan 1577                                                                            Mitsui -- DuPont                                                                           36,000    5.0                                                    Polychemical                                                                  K.K.                                                                Himilan 1652                                                                            Mitsui -- DuPont                                                                           24,000    5.0                                                    Polychemical                                                                  K.K.                                                                Himilan 1705                                                                            Mitsui -- DuPont                                                                           34,000    5.0                                                    Polychemical                                                                  K.K.                                                                Himilan 1702                                                                            Mitsui -- DuPont                                                                           22,000    14.0                                                   Polychemical                                                                  K.K.                                                                Surlyn 9650                                                                             E. I. DuPont de                                                                            32,000    5.0                                                    Nemours Co.                                                         Surlyn 9450                                                                             E. I. DuPont de                                                                            19,000    5.5                                                    Nemours Co.                                                         Surlyn 9950                                                                             E. I. DuPont de                                                                            37,000    5.5                                                    Nemours Co.                                                         Surlyn 9970                                                                             E. I. DuPont de                                                                            28,000    14.0                                                   Nemours Co.                                                         ______________________________________                                    

Another useful component (b) is an ethylene-unsaturated carboxylicacid-unsaturated carboxylate terpolymer. The unsaturated carboxylic acidand unsaturated carboxylate moieties are the same as in the sodium orzinc salt of an ethylene-unsaturated carboxylic acid-unsaturatedcarboxylate terpolymer used as main component (a) although theterpolymer as component (b) should have an M.F.R. of at least 4.Exemplary terpolymers are shown below.

                  TABLE 3                                                         ______________________________________                                                                Flexural                                                                      modulus   M.F.R.                                      Trade name Manufacturer (psi)     (g/10 min.)                                 ______________________________________                                        Nucrel AN 4307                                                                           Mitsui -- DuPont                                                                           3,100     15                                                     Polychemical                                                                  K.K.                                                               Nucrel AN 4212C                                                                          Mitsui -- DuPont                                                                           4,100     12                                                     Polychemical                                                                  K.K.                                                               Nucrel N 0825J                                                                           Mitsui -- DuPont                                                                           2,900     25                                                     Polychemical                                                                  K.K.                                                               ______________________________________                                    

Also included are mixtures of sodium and zinc salts ofethylene-unsaturated carboxylic acid copolymer, mixtures ofethylene-unsaturated carboxylic acid-unsaturated carboxylateterpolymers, and mixtures of a sodium or zinc salt ofethylene-unsaturated carboxylic acid copolymer with anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymer.

According to the present invention, components (a) and (b) are blended.The (total) amount of component (a) blended is 70 to 95% by weight ofthe resin components combined while the (total) amount of component (b)blended is 5 to 30% by weight of the resin components combined. If theamount of component (b) exceeds 30% by weight, peeling resistancebecomes low. If the amount of component (b) is less than 5%, the effectof improving moldability is not fully achieved. The preferred amount ofcomponent (b) blended is 10 to 20% by weight of the resin componentscombined.

In the cover composition of the invention, dyes, pigments (e.g.,titanium dioxide, zinc oxide and barium sulfate), UV absorbers,anti-oxidants, and dispersing aids (e.g., metal soaps) may be added tothe resin component consisting of components (a) and (b), if desired.These components are mixed in a conventional mixing means, for example,a closed kneading machine (Banbury mixer and kneader) and extruder.

The cover composition of the invention is molded and cured into a golfball cover by any conventional technique. The cover as cured should havea hardness of 55 to 80, preferably 65 to 75 on JIS C hardness scale anda tensile stress at 10% distortion of 40 to 140 kg/cm², preferably 70 to110 kg/cm². If the cover's 5 hardness is more than 80, hitting feel andspin property become poor. If the hardness is less than 55, the cover istoo soft to provide the ball with sufficient repulsion so that the balldoes not fly well. If the cover's tensile stress at 10% distortion isless than 40 kg/cm², the cover is less repulsive and susceptible todamages by club hitting. If the tensile stress at 10% distortion is morethan 120 kg/cm², the cover is likely to be peeled by iron shots.

It is to be noted that the tensile stress at 10% distortion is definedin JIS K-7113 as the tensile stress at which a specimen having a lengthof 100 is longitudinally stretched at a pulling rate of 500 mm/min. to alength of 110, the tensile stress being divided by the transversecross-sectional area of the specimen before the tensile test.

The cover composition of the invention may be used as covers for solidgolf balls such as two and three-piece golf balls and thread-wound golfballs. The cover composition exerts its function to the maximum whenused as solid golf ball covers.

EXAMPLE

Examples of the present invention are given below by way of illustrationand not by way of limitation.

Examples 1-5 & Comparative Examples 1-4

For the manufacture of large size two-piece golf balls, a core having adiameter of 38.5 mm was prepared by furnishing a rubber composition ofthe following formulation and effecting vulcanization at 155° C. for 18minutes.

    ______________________________________                                        Rubber composition (Core)                                                                       Parts by weight                                             ______________________________________                                        Polybutadiene rubber                                                                            100                                                         Barium sulfate    14.0                                                        Zinc oxide        5.0                                                         Anti-oxidant      0.2                                                         Zinc acrylate     36.0                                                        Peptizer          0.5                                                         Dicumyl peroxide  1.0                                                         ______________________________________                                    

Next, a cover composition was prepared by adding additives to the resincomposition shown in Table 4 so as to give the following formulation.The cover composition was injection molded over the core to form a coverof 2.0 mm thick thereon.

    ______________________________________                                                      Parts by weight                                                 ______________________________________                                        Resin composition                                                                             100                                                           Titanium oxide  5                                                             Magnesium stearate                                                                            1                                                             Bluing agent    0.03                                                          ______________________________________                                    

The resulting two-piece golf balls having a diameter of 42.7 mm and aweight of 45.2 grams were examined by the following tests.

(1) Dimple edge

When the cover composition was injection molded, it was examined how adimple pattern on the mold was transferred to the molded ball. The edgedefinition of dimples was visually observed.

(2) Sphericity

The outer diameter (mm) of a ball was measured at 15 points. Reported isa variation range R (mm) of the measured diameter. Higher valuesindicate less spherical balls.

(3) Hardness

Hardness is expressed by a distortion (mm) of a ball under a load of 100kg. Higher values indicate softer balls.

(4) Initial speed

An initial speed (m/s) was measured by an initial speed meter of thesame type as prescribed by USGA.

(5) Spin

With a #7 iron mounted on a swing robot, a ball was hit at a head speedof 36 m/s. The motion of the ball immediately after impact was analyzedusing high-speed photographs, determining a back spin quantity (rpm).

(6) Scuff test

A ball was kept at a temperature of 23° C. Three commercial pitchingwedges were mounted on a robot machine. The ball was hit at threepositions by the three pitching wedges, once at each position, at a headspeed of 37 m/s. The three hit areas were visually observed. Evaluationwas made according to the following criterion. The result is an averageof four balls tested.

    ______________________________________                                        Point       Remarks                                                           ______________________________________                                        10:         no change on the ball surface                                     9:          intermediate points 10 and 8                                      8:          very slight, substantially unperceivable                                      club face dent                                                    7:          intermediate points 8 and 6                                       6:          substantial club face dent, but no peeling                                    from the cover surface                                            5:          intermediate points 6 and 4                                       4:          perceivably peeled surface                                        3:          intermediate points 4 and 2                                       2:          peeled surface with some cracks                                   1:          outstanding cracks                                                ______________________________________                                         The results are shown in Table 4.                                        

                                      TABLE 4                                     __________________________________________________________________________                Example                  Comparative Example                                  1    2    3    4    5    1    2    3    4                         __________________________________________________________________________    Resin composition (pbw)                                                       H1706                                               20                        H1557       20                                 35                             H1855       30   40   70        50   50   10   15   30                        H1856                      90   25                                            S8120       50   50   20             50   50   50   50                        N0825J           10   10   10   25        40                                  Cover physical property                                                       N.F.R.      1.7  1.6  1.6  1.6  2.5  0.9  5.3  2.4  1.0                       JIS C hardness                                                                            74   72   71   74   66   68   60   79   79                        10% Tensile stress                                                                        100  80   77   81   65   62   52   120  124                       Golf ball configuration                                                       Dimple edge sharp                                                                              sharp                                                                              sharp                                                                              sharp                                                                              sharp                                                                              blunt                                                                              sharp                                                                              sharp                                                                              somewhat                                                                      blunt                     Sphericity  0.12 0.13 0.13 0.12 0.10 0.31 0.13 0.12 0.22                      Golf ball performance                                                         Hardness    2.66 2.75 2.72 2.77 2.87 2.85 2.95 2.58 2.58                      Initial speed                                                                             76.9 76.7 76.6 76.4 76.3 76.5 75.9 77.0 77.0                      Spin        8300 8600 8500 8200 8800 8600 9100 7700 7600                      Scuff       8    8    8    7    6    8    4    5    4                         __________________________________________________________________________

The cover composition of the invention is easily moldable and offersgolf balls having improved spin properties and resistant to impact byiron and other clubs.

Japanese Patent Application No. 6-37958 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

We claim:
 1. A composition for forming a golf ball cover, comprising(a)80 to 95% by weight of a sodium or zinc salt of an ethylene-unsaturatedcarboxylic acid-unsaturated carboxylate terpolymer or a mixture thereof,having a melt flow rate of less than 2 and (b) 20 to 5% by weight of anethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymerhaving a melt flow rate of at least 4, said composition curing into aproduct having a hardness of 55 to 80 on JIS C hardness scale and atensile stress at 10% distortion of 40 to 140 kg/cm².
 2. The compositionof claim 1 wherein the unsaturated carboxylic acid of components (a) and(b) has 3 to 8 carbon atoms and the unsaturated carboxylate is an esterof an unsaturated carboxylic acid having 3 to 8 carbon atoms with analcohol having 1 to 9 carbon atoms.
 3. A golf ball having a cover formedof the composition of claim 1 or
 2. 4. The composition of claim 1,wherein the melt flow rate of component (a) is not more than 1.0.
 5. Thecomposition of claim 1, wherein component (b) is present in an amount of20 to 10% by weight.
 6. The composition of claim 1, wherein thecomposition cures into a product having a hardness of 65 to 75 on theJIS C hardness scale and a tensile stress at 10% distortion of 70 to 110kg/cm².
 7. The composition of claim 1, wherein the sodium salt and zincsalt of component (a) are mixed in a weight ratio of from 20:80 to80:20.
 8. The composition of claim 7, wherein the sodium salt and zincsalt of component (a) are mixed in a weight ratio of from 40:60 to70:30.